1. Field of the Invention
The present invention generally relates to a high-frequency semiconductor integrated circuit, and more particularly to a high-frequency input impedance matching circuit and a high-frequency output impedance matching circuit in which a high-frequency power loss in an inductor is reduced.
2. Description of the Related Art
Recently, use of mobile telephones has widely spread. There is a demand to reduce the area of a high-frequency circuit which is used in the mobile telephones. To meet this demand, the high-frequency circuit is integrated into a large scale integration circuit (LSI). It is necessary for the high-frequency circuit to match its input and output impedance to 50xcexa9 so as to input and output a signal with high power efficiency.
FIG. 1 shows one type of a high-frequency input impedance matching circuit for a field effect transistor (FET) or a bipolar transistor. FIG. 2 shows another type of the high-frequency input impedance matching circuit.
A low pass filter (LPF) type high-frequency input impedance matching circuit as shown in FIG. 1 or a high pass filter (HPF) type high-frequency input impedance matching circuit as shown in FIG. 2 is generally used. In the LPF-type high-frequency input impedance matching circuit 105 as shown in FIG. 1, one terminal of an inductor 102 is connected to an input terminal of a transistor 101 and another terminal of the inductor 102 is connected to an input terminal 104. One terminal of a capacitor 103 is connected to the input terminal 104 and another terminal of the capacitor 103 is connected to a ground. An input signal is applied to the input terminal 104.
If the size of the transistor 101 is large, for example, an FET having a long gate or a bipolar transistor having a large emitter, the input impedance of the transistor is very low, for example, less than ten xcexa9. The high-frequency input impedance matching circuit 105 having the inductor 102 and the capacitor 103 as shown in FIG. 1 operates such that the input impedance of the input terminal 104 is equal to 50xcexa9.
On the other hand, in the HPF-type high-frequency input impedance matching circuit 205 as shown in FIG. 2, one terminal of a capacitor 202 is connected to the input terminal of the transistor 101 and another terminal of the capacitor 202 is connected to the input terminal 104. One terminal of an inductor 203 is connected to the input terminal 104 and another terminal of the inductor 203 is connected to the ground. An input signal is applied to the input terminal 104.
As mentioned above, if the size of the transistor 101 is large, the input impedance of the transistor is very low, for example, less than ten xcexa9. The high-frequency input impedance matching circuit 205 having the capacitor 202 and the inductor 203 as shown in FIG. 2 operates such that the input impedance of the input terminal 104 is equal to 50xcexa9.
Japanese Patent Application Number 57-45877 discloses a matching circuit for a microwave monolithic integration circuit (MMIC). In this application, a DC cut capacitor having a large capacitance is necessary. Therefore, to reduce the area of the DC cut capacitor in the MMIC, an improved impedance conversion circuit is utilized. The DC cut capacitor is included in the improved impedance conversion circuit to reduce the capacitance of the DC cut capacitor.
However, the prior art described above has the following problems.
One problem is that in the high-frequency input impedance matching circuit 105 as shown in FIG. 1, a power loss in the inductor connected to the gate of the FET 101 causes a degradation of characteristics of the high-frequency semiconductor circuit. FIG. 3 shows an equivalent circuit of the inductor 102. An inductor 301 shows an inductance component of the inductor 102 and a resistor 302 shows a series resistance component of the inductor 102.
In an MMIC, the inductor 102 is generally formed by a spiral inductor or a meander inductor. An inductor having a long length is generally necessary to form an inductor having a large inductance. The inductor having the long length also has the resistance component 302 having a high resistance. Furthermore, when a frequency of the input signal applied to the input terminal 104 is increased, the resistance of the resistance component 302 is also increased by a skin effect.
As a result, when the resistance of the resistance component 302 is higher than the input impedance of the FET 101, much of the high-frequency power supplied to the input terminal 104 is consumed by the series resistance component 302 in the inductor 102. This means that high-frequency power loss is increased. Therefore, in the high-frequency input impedance matching circuit, it is necessary to reduce the high-frequency power loss in the inductor which is connected to the FET 101 having the small input impedance.
Next, in the HPF-type high-frequency input impedance matching circuit 205 as shown in FIG. 2, a power loss in the inductor which is connected between one terminal of the capacitor 202 and the ground also causes a degradation of characteristics of the high-frequency semiconductor circuit.
In the HPF-type high-frequency input impedance matching circuit 205, the capacitor 202 is connected to the gate of the FET 101. The high frequency power loss in the capacitor which is used in the high-frequency circuit is generally lower than that of the inductor used in the high-frequency circuit. When the capacitor 202 has a large capacitance value, an impedance at a point 206 is almost the same value as that of the input impedance of the FET 101. Therefore, it is also necessary to reduce the high-frequency power loss in the inductor 203.
However, the inductor 203 causes the high-frequency power loss for the same reason as the LPF-type high-frequency input impedance matching circuit. Then, conventionally, the inductor 203 is formed outside the chip by a bonding wire, a package lead, a printed wire or a chip inductor.
When the inductor 203 is formed outside the chip, the following problem occurs. Some kinds of characteristics of the high-frequency circuit fluctuate according to the inductance of the inductor 203. Therefore, when the inductor 203 is formed outside the chip, it is difficult to consistently produce the high-frequency circuit with stable characteristics because the inductance has a different value for each inductor 203 formed outside the chip. When the frequency of the input signal applied to the input terminal 104 is increased, an inductor having a small inductance value is necessary. However, it is difficult to produce the inductor 203 having a small inductance value outside the chip with high-precision.
The problems for only the high-frequency input impedance matching circuits are described above. However, the high-frequency output impedance matching circuits also have the same problems as mentioned above.
It is a general object of the present invention to provide a high-frequency input impedance matching circuit, a high-frequency output impedance matching circuit and a semiconductor integrated circuit in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a high-frequency input impedance matching circuit and a high-frequency output impedance matching circuit in which the high-frequency power loss in an inductor is reduced.
The above objects of the present invention are achieved by a high-frequency input impedance matching circuit having an inductor which is connected between an input of a transistor and an input terminal of the high-frequency input impedance matching circuit, a first capacitor one end of which is connected to the input terminal and another end of which is connected to a ground and a second capacitor which is connected to the inductor in parallel.
According to the invention, the effective reactance of the inductor is increased by connecting the second capacitor to the inductor in parallel. Therefore, an inductor having a small inductance value can be used. As a result, the high-frequency power loss in the high-frequency input impedance matching circuit can be reduced because the inductor having a short length can be used. The high-frequency input impedance matching circuit according to the present invention can also operate such that the input impedance of the input terminal 104 is equal to 50xcexa9.